Technique for forming a cathode of a wet electrolytic capacitor

ABSTRACT

A technique for forming a cathode of a wet electrolytic capacitor is provided. The cathode contains a metal substrate having a roughened surface and a conductive coating that contains a substituted polythiophene. The degree of surface contact between the conductive coating and the roughened surface is enhanced in the present invention by selectively controlling the manner in which the conductive coating is formed. More particularly, the conductive coating is formed by applying a precursor solution to the roughened surface that includes both a precursor thiophene monomer and an oxidative catalyst. Contrary to techniques in which either the monomer or catalyst is applied separately and initially contacts the metal surface, the presence of the monomer and catalyst within the same solution allows polymer chains to grow immediately adjacent to the surface of the metal substrate and within the pits. This can significantly increase the degree of contact between the conductive coating and metal substrate, thereby resulting in improved mechanical robustness and electrical performance (e.g., reduced equivalent series resistance and leakage current). To minimize the likelihood of premature polymerization, the oxidative catalyst is also employed in an amount less than what is required to fully react all of the reagent assuming 100% yield (i.e., “stoichiometric amount”). This can slow the polymerization of the monomer, creating oligomers that are shorter than if fully polymerized, which can better penetrate into the roughened regions of the metal substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional ApplicationSer. No. 61/383,445 having a filing date of Sep. 16, 2010.

BACKGROUND OF THE INVENTION

Wet capacitors are increasingly being used in the design of circuits dueto their volumetric efficiency, reliability, and process compatibility.Wet capacitors typically have a larger capacitance per unit volume thancertain other types of capacitors, making them valuable in high-current,high power and low-frequency electrical circuits. One type of wetcapacitor that has been developed is a wet electrolytic capacitor thatincludes a valve metal anode, a cathode, and a liquid electrolyte. Theunit cell voltage in this type of capacitor is generally higher due tothe formation of a dielectric metal oxide film over the anode surface.Wet electrolytic capacitors tend to offer a good combination of highcapacitance with low leakage current. Another type of wet capacitor is awet symmetric capacitor in which the anode and cathode are similar interms of structure and composition. The unit cell voltage in this typeof capacitor is generally low due to the inevitable decomposition of theelectrolyte at high voltage. Whether electrolytic or symmetric, however,the cathodes of wet capacitors typically include a substrate and acoating that provides high capacitance through a faradic or non-faradicmechanism. Conventional coatings include activated carbon, metal oxides(e.g., ruthenium oxide), and the like. Unfortunately, however, thecoatings can become easily detached under certain conditions, such as inthe presence of aqueous electrolytes.

As such, a need remains for a high voltage wet electrolytic capacitorthat possesses good mechanical robustness and electrical performance.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method forforming a cathode of a wet electrolytic capacitor is disclosed. Themethod comprises applying a precursor solution to a roughened surface ofa metal substrate, wherein the precursor solution contains a substitutedthiophene monomer and an oxidative catalyst and the oxidative catalystis employed in an amount less than the stoichiometric amount. Thesubstituted thiophene monomer is polymerized to form a coating thatcontains an intrinsically conductive substituted polythiophene.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a cross-sectional view of one embodiment of the wetelectrolytic capacitor of the present invention; and

FIG. 2 is a cross-sectional view of a micro-roughened metal substratethat may be employed in one embodiment of the present invention.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

Generally speaking, the present invention is directed to a technique forforming a cathode of a wet electrolytic capacitor. The cathode containsa metal substrate having a roughened surface and a conductive coatingthat contains a substituted polythiophene. The degree of surface contactbetween the conductive coating and the roughened surface is enhanced inthe present invention by selectively controlling the manner in which theconductive coating is formed. More particularly, the conductive coatingis formed by applying a precursor solution to the roughened surface thatincludes both a precursor thiophene monomer and an oxidative catalyst.Contrary to techniques in which either the monomer or catalyst isapplied separately and initially contacts the metal surface, thepresence of the monomer and catalyst within the same solution allowspolymer chains to grow immediately adjacent to the surface of the metalsubstrate and within the pits. This can significantly increase thedegree of contact between the conductive coating and metal substrate,thereby resulting in improved mechanical robustness and electricalperformance (e.g., reduced equivalent series resistance and leakagecurrent). To minimize the likelihood of premature polymerization, theoxidative catalyst is also employed in an amount less than what isrequired to fully react all of the reagent assuming 100% yield (i.e.,“stoichiometric amount”). This can slow the polymerization of themonomer, creating oligomers that are shorter than if fully polymerized,which can better penetrate into the roughened regions of the metalsubstrate.

Various embodiments of the present invention will now be described inmore detail.

I. Cathode

A. Metal Substrate

The metal substrate of the cathode may include any metal, such astantalum, niobium, aluminum, nickel, hafnium, titanium, copper, silver,steel (e.g., stainless), alloys thereof (e.g., electrically conductiveoxides), composites thereof (e.g., metal coated with electricallyconductive oxide), and so forth. Titanium and tantalum, as well asalloys thereof, are particularly suitable for use in the presentinvention. The geometric configuration of the substrate may generallyvary as is well known to those skilled in the art, such as in the formof a container, can, foil, sheet, screen, mesh, etc. In one embodiment,for example, the metal substrate forms a casing having a generallycylindrical shape. It should be understood, however, that any geometricconfiguration may be employed in the present invention, such asD-shaped, rectangular, triangular, prismatic, etc. The casing mayoptionally include a lid that covers the anode and electrolyte, whichmay be formed from the same or different material than the casing.

Regardless of its particular form, the substrate is roughened toincrease its surface area and increase the degree to which theconductive polymer is able to adhere thereto. In this regard, a varietyof techniques may generally be employed to roughen the surface of themetal substrate. In one embodiment, for example, the surface ischemically etched, such as by applying a solution of a corrosivesubstance (e.g., hydrochloric acid) to the surface. The surface may alsobe electrochemically etched, such as by applying a voltage to a solutionof the corrosive substance so that it undergoes electrolysis. Thevoltage may be raised to a sufficiently high level to initiate“sparking” at the surface of the substrate, which is believed to createhigh local surface temperatures sufficient that etches away thesubstrate. This technique is described in more detail in U.S. PatentApplication Publication No. 2010/0142124 to Dreissig, et al., which isincorporated herein in its entirety by reference thereto for allpurposes.

In addition to chemical or electrochemical roughening techniques,mechanical roughening may also be employed. In one embodiment, forinstance, the surface of the metal substrate may be abrasive blasted bypropelling a stream of abrasive media against at least a portion of asurface thereof. Among other things, this physically stresses anddeforms the surface to create small pits on the surface so that itbecomes pitted and roughened. These pits can increase the degree towhich the conductive polymer is able to adhere to the metal substrate.Further, abrasive blasting may distribute the pits in a substantiallyuniform manner so that the surface is generally smooth on a macroscopiclevel. The surface area of the substrate may also be increased. Forinstance, the area of the surface of the substrate prior to rougheningmay range from about 0.05 to about 5 square centimeters, in someembodiments from about 0.1 to about 3 square centimeters, and in someembodiments, from about 0.5 to about 2 square centimeters. The ratio ofthe area of the micro-roughened surface to that of the initial surface(prior to micro-roughening) may likewise be from about 1 to about 5, andin embodiments, from about 1.1 to about 3. The increase in surface areacan allow for increased cathode capacitance for a given size and/orcapacitors with a reduced size for a given capacitance.

Referring to FIG. 2, for example, one embodiment of a metal substrate200 is shown that has been abrasive blasted to form a micro-roughenedsurface 204 having a plurality of pits 206. The relative size andspacing of the pits 206 may vary depending on the desired properties forthe capacitor. For example, the average depth (“D”) of the pits 206 maybe from about 200 to about 2500 nanometers, in some embodiments fromabout 300 to about 2000 nanometers, and in some embodiments, from about500 to about 1500 nanometers. Likewise, adjacent pits 206 may be spacedapart a “peak-to-peak” distance (“P”) that ranges from about 20 to about500 micrometers, in some embodiments from about 30 to about 400micrometers, in some embodiments, from about 50 to about 200micrometers. The number of pits 206 may also be high enough to producethe desired increase in surface area. For example, the surface maypossess from 1 to 20, in some embodiments, from 2 to 15, and in someembodiments, from 3 to 10 pits per 100 square micrometers. The pits 206may be disposed uniformly or non-uniformly across the surface 202. Forexample, the pits 206 may be present in a spaced-apart fashion over thesurface so that they form “island-like” structures. It should beunderstood that the entire surface of the substrate need not be abraded.In fact, in certain embodiments, it may be desired to only abrade aportion of the metal substrate so that the remaining portion isrelatively smooth for attaching a sealing mechanism. For example, aportion of the substrate may be covered by a masking device (e.g.,ferrule, tape, etc.) during abrasion so that the pits are formed only inthe desired locations. When employing a cylindrical substrate, forinstance, it may be desirable to use a generally cylindrical, hollowferrule to mask a top portion of the substrate.

The methods employed to abrasively blast the surface may be selectivelycontrolled to achieve the desired features. Suitable methods mayinclude, for example, sandblasting, bead blasting, pellet blasting, etc.The abrasive media employed in such methods may vary and include, forexample, ceramic particles, metal particles, polymeric particles,liquids (e.g., water), etc. Sandblasting is particularly suitable foruse in the present invention and generally involves propelling a streamof ceramic media (e.g., silicon carbide, aluminum oxide, titaniumdioxide, etc.) through a nozzle and against the surface of thesubstrate. The size of the abrasive media may be selected based on thetype of substrate, the pressure employed, and the desired qualities ofthe finished substrate. For example, the abrasive media may have anaverage size of from about 20 micrometers to about 150 micrometers.Further, the pressure and time that the abrasive media is propelledtoward the surface may range from about 1 to about 50 pounds per squareinch, and in some embodiments from about 10 to about 35 pounds persquare inch, for a time period of from about 1 to about 50 seconds, insome embodiments from about 5 to about 40 seconds, and in someembodiments, from about 10 to about 30 seconds. At such conditions, thedistance that the injection nozzle is spaced from the surface of themetal substrate may also be controlled to achieve the desired pitformation, such as from about 0.1 to about 5 inches from the surface ofthe substrate. The nozzle may be stationary or it may be moved relativeto the substrate during application of the abrasive media. When blastingthe interior surface of a cylindrical casing, for example, the nozzlemay be rotated or remain stationary while the casing is rotated. One ormore blasting steps may generally be employed. Once complete, anyabrasive media remaining on the surface of the metal substrate istypically removed, such as by washing the substrate.

B. Conductive Coating

As indicated above, a conductive coating is formed on themicro-roughened surface of the metal substrate. The conductive coatingcontains a substituted polythiophene, which is r-conjugated and hasintrinsic electrical conductivity (e.g., electrical conductivity of atleast about 1 μS cm⁻¹). In one particular embodiment, the substitutedpolythiophene has recurring units of general formula (I), formula (II),or both:

wherein,

A is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R is a linear or branched, optionally substituted C₁ to C₁₈ alkylradical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- ortert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl,n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical(e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononylcyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkylradical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-,3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkylradical, or hydroxyl radical; and

x is an integer from 0 to 8, in some embodiments, from 0 to 2, and insome embodiments, x is 0. Example of substituents for the radicals “A”or “R” include, for instance, alkyl, cycloalkyl, aryl, aralkyl, alkoxy,halogen, ether, thioether, disulphide, sulfoxide, sulfone, sulfonate,amino, aldehyde, keto, carboxylic acid ester, carboxylic acid,carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups,carboxylamide groups, and so forth.

The total number of recurring units of general formula (I) or formula(II) or of general formulae (I) and (II) is typically from 2 to 2,000,and in some embodiments, from 2 to 100.

Particularly suitable substituted polythiophenes are those in which “A”is an optionally substituted C₂ to C₃ alkylene radical and x is 0 or 1.In one particular embodiment, the substituted polythiophene ispoly(3,4-ethylenedioxythiophene) (“PEDT”), which has recurring units offormula (II), wherein “A” is CH₂—CH₂ and “x” is 0. The monomers used toform such polymers may vary as desired. For instance, particularlysuitable monomers are substituted 3,4-alkylenedioxythiophenes having thegeneral formula (III), (IV), or both:

wherein, A, R, and X are as defined above.

Examples of such monomers include, for instance, optionally substituted3,4-ethylenedioxythiophenes. One commercially suitable example of3,4-ethylenedioxthiophene is available from H.C. Starck GmbH under thedesignation Clevios™ M. Derivatives of these monomers may also beemployed that are, for example, dimers or trimers of the above monomers.Higher molecular derivatives, i.e., tetramers, pentamers, etc. of themonomers are suitable for use in the present invention. The derivativesmay be made up of identical or different monomer units and used in pureform and in a mixture with one another and/or with the monomers.Oxidized or reduced forms of these precursors may also be employed.

The thiophene monomers, such as described above, are chemicallypolymerized in the presence of an oxidative catalyst. The oxidativecatalyst typically includes a transition metal cation, such asiron(III), copper(II), chromium(VI), cerium(IV), manganese(IV),manganese(VII), ruthenium(III) cations, etc. A dopant may also beemployed to provide excess charge to the conductive polymer andstabilize the conductivity of the polymer. The dopant typically includesan inorganic or organic anion, such as an ion of a sulfonic acid. Incertain embodiments, the oxidative catalyst employed in the precursorsolution has both a catalytic and doping functionality in that itincludes a cation (e.g., transition metal) and anion (e.g., sulfonicacid). For example, the oxidative catalyst may be a transition metalsalt that includes iron(III) cations, such as iron(III) halides (e.g.,FeCl₃) or iron(III) salts of other inorganic acids, such as Fe(ClO₄)₃ orFe₂(SO₄)₃ and the iron(III) salts of organic acids and inorganic acidscomprising organic radicals. Examples of iron (III) salts of inorganicacids with organic radicals include, for instance, iron(III) salts ofsulfuric acid monoesters of C₁ to C₂₀ alkanols (e.g., iron(III) salt oflauryl sulfate). Likewise, examples of iron(III) salts of organic acidsinclude, for instance, iron(III) salts of C₁ to C₂₀ alkane sulfonicacids (e.g., methane, ethane, propane, butane, or dodecane sulfonicacid); iron (III) salts of aliphatic perfluorosulfonic acids (e.g.,trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, orperfluorooctane sulfonic acid); iron (III) salts of aliphatic C₁ to C₂₀carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (III) saltsof aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid orperfluorooctane acid); iron (III) salts of aromatic sulfonic acidsoptionally substituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonicacid, o-toluene sulfonic acid, p-toluene sulfonic acid, ordodecylbenzene sulfonic acid); iron (III) salts of cycloalkane sulfonicacids (e.g., camphor sulfonic acid); and so forth. Mixtures of theseabove-mentioned iron(III) salts may also be used. Iron(III)-p-toluenesulfonate, iron(III)-o-toluene sulfonate, and mixtures thereof, areparticularly suitable. One commercially suitable example ofiron(III)-p-toluene sulfonate is available from H.C. Starck GmbH underthe designation Clevios™ C.

Prior to application to the metal substrate, a precursor solution isformed that contains both the monomeric thiophene and the oxidativecatalyst. Among other things, this allows polymer chains to growimmediately adjacent to the surface of the metal substrate. To minimizethe likelihood of premature polymerization, the oxidative catalyst isemployed in an amount less than what is required to fully react all ofthe reagent assuming 100% yield (i.e., “stoichiometric amount”). Thiscan slow the polymerization of the monomer, creating oligomers that areshorter than if fully polymerized, which can better penetrate into theroughened regions of the metal substrate. For example, thestoichiometric amount of the catalyst can be about 2 moles catalyst per1 mole of monomer. In such cases, the oxidative catalyst may be employedin the present invention in an amount of less than about 2 moles ofcatalyst per 1 mole of monomer, in some embodiments less than about 1.5moles of catalyst per 1 mole of monomer, in some embodiments from about0.4 to about 1.5 moles of catalyst per 1 mole of monomer, and in someembodiments, from about 0.8 to about 1.4 moles of catalyst per 1 mole ofmonomer. The ratio of the weight of catalyst to the weight of monomermay likewise be less than about 20, in some embodiments less than about15, in some embodiments from about 4 to about 15, and in someembodiments, from about 8 to about 14.

Premature polymerization may also be inhibited through the use of areaction inhibitor. In one particular embodiment, for example, a polaraprotic solvent is employed as a reaction inhibitor. The localizedcharge on the aprotic solvent can attract, through electron donation(e.g., acid-base reactions), the positively charged metal (e.g., iron)of the oxidative catalyst to form a weakly bonded complex. This weakcomplex can inhibit the ability of the catalyst to oxidize the monomerfor polymerization. As such, only a relatively small amount, if any, ofthe monomer is prematurely polymerized in the precursor solution priorto its application to the metal substrate. Additionally, such solventscan also dissolve oligomers that prematurely form while still in theprecursor solution. Thus, the oligomers can be inhibited from furtherpolymerization. Particularly suitable polar aprotic solvents include,but are not limited to, N-methylpyrrolidone, dimethyl sulfoxide,dimethylformamide, acetonitrile, hexamethylphosphorotriamide, dimethylacetamide, methyl ethyl ketone, acetone, ethyl acetate, tetrahydrofuran,dichloromethane, and so forth.

In addition to aprotic solvents, other solvents (e.g., polar protic ornon-polar) may also be employed in the solution, either alone or as aco-solvent with the aprotic solvent. Examples of suitable solventsinclude, for instance, glycols (e.g., propylene glycol, butylene glycol,triethylene glycol, hexylene glycol, polyethylene glycols,ethoxydiglycol, and dipropyleneglycol); glycol ethers (e.g., methylglycol ether, ethyl glycol ether, and isopropyl glycol ether); ethers(e.g., diethyl ether and tetrahydrofuran); alcohols (e.g., methanol,ethanol, n-propanol, iso-propanol, and butanol); triglycerides; ketones;esters (e.g., ethyl acetate, butyl acetate, diethylene glycol etheracetate, and methoxypropyl acetate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile,butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethylsulfoxide (DMSO) and sulfolane); and so forth. Particularly suitablesolvents are aliphatic alcohols, such as ethanol, propanol, methanol,isopropanol, butanol, and so forth.

The total amount of solvents in the precursor solution may be from about40 wt. % wt. % to about 90 wt. %, in some embodiments from about 50 wt.% to about 85 wt. %, and in some embodiments, from about 60 wt. % toabout 80 wt. %. For instance, polar protic and/or non-polar solventstypically constitute from about 30 wt. % to about 90 wt. %, in someembodiments, and in some embodiments, from about 50 wt. % to about 80wt. % of the precursor solution. Likewise, when employed, polar aproticsolvents typically constitute from about 0.001 wt. % to about 10 wt. %,and in some embodiments, from about 0.05 wt. % to about 1 wt. % of theprecursor solution. The oxidative catalysts may also be employed in anamount from about 5 wt. % to about 50 wt. %, in some embodiments fromabout 10 to about 40 wt. %, and in some embodiments, from about 15 toabout 30 wt. %, of the precursor solution, while the monomers mayconstitute from about 0.1 wt. % to about 10 wt. %, in some embodimentsfrom about 0.5 wt. % to about 8 wt. %, and in some embodiments, fromabout 1 wt. % to about 6 wt. %, of the precursor solution

Once the solution is formed, it may be applied to the metal substrateusing any known technique. For instance, suitable techniques may includedipping, spin coating, impregnation, pouring, dropwise application,injection, spraying, doctor blading, brushing or printing (e.g.,ink-jet, screen, or pad printing). For example, the metal substrate maybe dipped into or sprayed with the precursor solution. Although it mayvary depending on the application technique employed, the viscosity ofthe solution is typically from about 0.1 to about 100,000 mPas (measuredat a shear rate of 100 s⁻¹), in some embodiments from about 1 to about10,000 mPas, in some embodiments from about 10 to about 1,500 mPas, andin some embodiments, from about 100 to about 1000 mPas.

Prior to and/or during application to the metal substrate, thetemperature of the precursor solution may be selectively controlled tohelp stabilize the solution and inhibit premature polymerization. Forexample, the precursor solution may be maintained at a temperature ofless than about 20° C., in some embodiments from about −10° C. to about15° C., and in some embodiments, from about 0° C. to about 10° C. Onceapplied to the surface of the substrate, its temperature may be raised,such as to room temperature (e.g., 25° C.) or higher, in someembodiments from about 25° C. to about 250° C., to allow polymerizationto proceed and form a conductive polymer coating thereon. If desired,the above-described application steps may be repeated until the desiredthickness of the coating is achieved. In some embodiments, only arelatively thin layer of the coating is formed at a time. The totaltarget thickness of the coating may generally vary depending on thedesired properties of the capacitor. Typically, the resulting conductivepolymer coating has a thickness of from about 0.2 micrometers (“μm”) toabout 50 μm, in some embodiments from about 0.5 μm to about 20 μm, andin some embodiments, from about 1 μm to about 5 μm. It should beunderstood that the thickness of the coating is not necessarily the sameat all locations on the metal substrate. Nevertheless, the averagethickness of the coating on the substrate generally falls within theranges noted above.

The conductive polymer coating may optionally be healed. Healing mayoccur after each application of a conductive polymer layer or may occurafter the application of the entire conductive polymer coating. In someembodiments, the conductive polymer can be healed by dipping the metalsubstrate into an electrolyte solution, and thereafter applying aconstant voltage to the solution until the current is reduced to apreselected level. If desired, such healing can be accomplished inmultiple steps. For example, an electrolyte solution can be a dilutesolution of the monomer, the catalyst, and dopant in an alcohol solvent(e.g., ethanol). The coating may also be washed if desired to removevarious byproducts, excess reagents, and so forth.

Without intending to be limited by theory, it is believed that chargingof the capacitor to a high voltage (e.g., greater than the formationvoltage) forces ions of the electrolyte into coatings containing suchsubstituted polythiophenes. This causes the conductive polymer to“swell” and retain the ions near the surface, thereby enhancing chargedensity. Because the polymer is generally amorphous and non-crystalline,it can also dissipate and/or absorb the heat associated with the highvoltage. Upon discharge, it is also believed that the substitutedpolythiophene “relaxes” and allows ions in the electrolyte to move outof the coating. Through such swelling and relaxation mechanism, chargedensity near the metal substrate can be increased without a chemicalreaction with the electrolyte. Accordingly, one beneficial aspect of thepresent invention is that mechanical robustness and good electricalperformance may be provided without the need for conventional conductivecoatings, such as those made from activated carbon or metal oxides(e.g., ruthenium oxide). In fact, the present inventors have discoveredthat excellent results may be achieved using the coating as theprincipal material on the metal substrate. That is, the coating mayconstitute at least about 90 wt. %, in some embodiments at least about92 wt. %, and in some embodiments, at least about 95 wt. % of thematerial(s) present on the metal substrate. Nevertheless, it should beunderstood that other conductive coatings may also be used in someembodiments of the present invention.

II. Anode

The anode of the electrolytic capacitor includes a porous body that maybe formed from a valve metal composition. The specific charge of thecomposition may vary. In certain embodiments, for example, compositionshaving a high specific charge are employed, such as about 5,000 μF*V/gor more, in some embodiments about 25,000 μF*V/g or more, in someembodiments about 40,000 μF*V/g or more, and in some embodiments, fromabout 70,000 to about 300,000 μF*V/g. The valve metal compositioncontains a valve metal (i.e., metal that is capable of oxidation) orvalve metal-based compound, such as tantalum, niobium, aluminum,hafnium, titanium, alloys thereof, oxides thereof, nitrides thereof, andso forth. For example, the valve metal composition may contain anelectrically conductive oxide of niobium, such as niobium oxide havingan atomic ratio of niobium to oxygen of 1:1.0±1.0, in some embodiments1:1.0±0.3, in some embodiments 1:1.0±0.1, and in some embodiments,1:1.0±0.05. For example, the niobium oxide may be NbO_(0.7), NbO_(1.0),NbO_(1.1), and NbO₂. Examples of such valve metal oxides are describedin U.S. Pat. No. 6,322,912 to Fife; U.S. Pat. No. 6,391,275 to Fife etal.; U.S. Pat. No. 6,416,730 to Fife et al.; U.S. Pat. No. 6,527,937 toFife; U.S. Pat. No. 6,576,099 to Kimmel, et al.; U.S. Pat. No. 6,592,740to Fife, et al.; and U.S. Pat. No. 6,639,787 to Kimmel, et al.; and U.S.Pat. No. 7,220,397 to Kimmel, et al., as well as U.S. Patent ApplicationPublication Nos. 2005/0019581 to Schnitter; 2005/0103638 to Schnitter,et al.; 2005/0013765 to Thomas, et al., all of which are incorporatedherein in their entirety by reference thereto for all purposes.

Conventional fabricating procedures may generally be utilized to formthe porous anode body. In one embodiment, a tantalum or niobium oxidepowder having a certain particle size is first selected. The particlesmay be flaked, angular, nodular, and mixtures or variations thereof. Theparticles also typically have a screen size distribution of at leastabout 60 mesh, in some embodiments from about 60 to about 325 mesh, andin some embodiments, from about 100 to about 200 mesh. Further, thespecific surface area is from about 0.1 to about 10.0 m²/g, in someembodiments from about 0.5 to about 5.0 m²/g, and in some embodiments,from about 1.0 to about 2.0 m²/g. The term “specific surface area”refers to the surface area determined by the physical gas adsorption(B.E.T.) method of Bruanauer, Emmet, and Teller, Journal of AmericanChemical Society, Vol. 60, 1938, p. 309, with nitrogen as the adsorptiongas. Likewise, the bulk (or Scott) density is typically from about 0.1to about 5.0 g/cm³, in some embodiments from about 0.2 to about 4.0g/cm³, and in some embodiments, from about 0.5 to about 3.0 g/cm³.

To facilitate the construction of the anode body, other components maybe added to the electrically conductive particles. For example, theelectrically conductive particles may be optionally mixed with a binderand/or lubricant to ensure that the particles adequately adhere to eachother when pressed to form the anode body. Suitable binders may includecamphor, stearic and other soapy fatty acids, Carbowax (Union Carbide),Glyptal (General Electric), polyvinyl alcohols, naphthalene, vegetablewax, and microwaxes (purified paraffins). The binder may be dissolvedand dispersed in a solvent. Exemplary solvents may include water,alcohols, and so forth. When utilized, the percentage of binders and/orlubricants may vary from about 0.1% to about 8% by weight of the totalmass. It should be understood, however, that binders and lubricants arenot required in the present invention.

The resulting powder may be compacted using any conventional powderpress mold. For example, the press mold may be a single stationcompaction press using a die and one or multiple punches. Alternatively,anvil-type compaction press molds may be used that use only a die andsingle lower punch. Single station compaction press molds are availablein several basic types, such as cam, toggle/knuckle and eccentric/crankpresses with varying capabilities, such as single action, double action,floating die, movable platen, opposed ram, screw, impact, hot pressing,coining or sizing. If desired, any binder/lubricant may be removed aftercompression, such as by heating the formed pellet under vacuum at acertain temperature (e.g., from about 150° C. to about 500° C.) forseveral minutes. Alternatively, the binder/lubricant may also be removedby contacting the pellet with an aqueous solution, such as described inU.S. Pat. No. 6,197,252 to Bishop, et al., which is incorporated hereinin its entirety by reference thereto for all purposes.

The size of the pressed anode body may depend in part on the desiredsize of the metal substrate. In certain embodiments, the length of theanode body may range from about 1 to about 100 millimeters, in someembodiments from about 5 to about 60 millimeters, and in someembodiments, from about 5 to about 20 millimeters. The width (ordiameter) of the anode body may also range from about 0.5 to about 20millimeters, in some embodiments from about 1 to about 20 millimeters,and in some embodiments, from about 4 to about 10 millimeters. The shapeof the anode body may also be selected to improve the electricalproperties of the resulting capacitor. For example, the anode body mayhave a shape that is cylindrical, rectangular, D-shaped, curved, etc.

The anode body may be anodically oxidized (“anodized”) so that adielectric layer is formed over and/or within the anode. For example, atantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅).Typically, anodization is performed by initially applying a solution tothe anode, such as by dipping anode into the electrolyte. A solvent isgenerally employed, such as water (e.g., deionized water). To enhanceionic conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs typically rangesfrom about 4 to about 250 V, and in some embodiments, from about 9 toabout 200 V, and in some embodiments, from about 20 to about 150 V.During oxidation, the anodizing solution can be kept at an elevatedtemperature, such as about 30° C. or more, in some embodiments fromabout 40° C. to about 200° C., and in some embodiments, from about 50°C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

III. Electrolyte

The electrolyte is the electrically active material that provides theconnecting path between the anode and cathode. Various suitableelectrolytes are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 toEvans, et al., which are incorporated herein their entirety by referencethereto for all purposes. Typically, the electrolyte is ionicallyconductive in that has an ionic conductivity of from about 0.5 to about1000 milliSiemens per centimeter (“mS/cm”), in some embodiments fromabout 1 to about 100 mS/cm, in some embodiments from about 5 mS/cm toabout 100 mS/cm, and in some embodiments, from about 10 to about 50mS/cm, determined at a temperature of 25° C. using any known electricconductivity meter (e.g., Oakton Con Series 11). Within the ranges notedabove, it is believed that the ionic conductivity of the electrolyteallows the electric field to extend into the electrolyte to a length(Debye length) sufficient to result in significant charge separation.This extends the potential energy of the dielectric to the electrolyteso that the resulting capacitor is able to store even more potentialenergy than predicted by the thickness of the dielectric. In otherwords, the capacitor may be charged to a voltage that exceeds theformation voltage of the dielectric. The ratio of the voltage to whichthe capacitor can be charged to the formation voltage may, for instance,be from about 1.0 to 2.0, in some embodiments from about 1.1 to about1.8, and in some embodiments, from about 1.2 to about 1.6. As anexample, the voltage to which the capacitor is charged may be from about200 to about 350 V, in some embodiments from about 220 to about 320 V,and in some embodiments, from about 250 to about 300V.

The electrolyte is generally in the form of a liquid, such as a solution(e.g., aqueous or non-aqueous), precursor solution, gel, etc. Forexample, the working electrolyte may be an aqueous solution of an acid(e.g., sulfuric acid, phosphoric acid, or nitric acid), base (e.g.,potassium hydroxide), or salt (e.g., ammonium salt, such as a nitrate),as well any other suitable electrolyte known in the art, such as a saltdissolved in an organic solvent (e.g., ammonium salt dissolved in aglycol-based solution). Various other electrolytes are described in U.S.Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al., which areincorporated herein their entirety by reference thereto for allpurposes.

The desired ionic conductivity may be achieved by selecting ioniccompound(s) (e.g., acids, bases, salts, and so forth) within certainconcentration ranges. In one particular embodiment, salts of weakorganic acids may be effective in achieving the desired conductivity ofthe electrolyte. The cation of the salt may include monatomic cations,such as alkali metals (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺), alkaline earthmetals (e.g., Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), transition metals (e.g.,Ag⁺, Fe²⁺, Fe³⁺, etc.), as well as polyatomic cations, such as NH₄ ⁺.The monovalent ammonium (NH₄ ⁺), sodium (Na⁺), and lithium (Li⁺) areparticularly suitable cations for use in the present invention. Theorganic acid used to form the anion of the salt is “weak” in the sensethat it typically has a first acid dissociation constant (pK_(a1)) ofabout 0 to about 11, in some embodiments about 1 to about 10, and insome embodiments, from about 2 to about 10, determined at 25° C. Anysuitable weak organic acids may be used in the present invention, suchas carboxylic acids, such as acrylic acid, methacrylic acid, malonicacid, succinic acid, salicylic acid, sulfosalicylic acid, adipic acid,maleic acid, malic acid, oleic acid, gallic acid, tartaric acid (e.g.,dextotartaric acid, mesotartaric acid, etc.), citric acid, formic acid,acetic acid, glycolic acid, oxalic acid, propionic acid, phthalic acid,isophthalic acid, glutaric acid, gluconic acid, lactic acid, asparticacid, glutaminic acid, itaconic acid, trifluoroacetic acid, barbituricacid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoicacid, etc.; blends thereof, and so forth. Polyprotic acids (e.g.,diprotic, triprotic, etc.) are particularly desirable for use in formingthe salt, such as adipic acid (pK_(a1) of 4.43 and pK_(a2) of 5.41),α-tartaric acid (pK_(a1) of 2.98 and pK_(a2) of 4.34), meso-tartaricacid (pK_(a1) of 3.22 and pK_(a2) of 4.82), oxalic acid (pK_(a1) of 1.23and pK_(a2) of 4.19), lactic acid (pK_(a1) of 3.13, pK_(a2) of 4.76, andpK_(a3) of 6.40), etc.

While the actual amounts may vary depending on the particular saltemployed, its solubility in the solvent(s) used in the electrolyte, andthe presence of other components, such weak organic acid salts aretypically present in the electrolyte in an amount of from about 0.1 toabout 25 wt. %, in some embodiments from about 0.2 to about 20 wt. %, insome embodiments from about 0.3 to about 15 wt. %, and in someembodiments, from about 0.5 to about 5 wt. %.

The electrolyte is typically aqueous in that it contains an aqueoussolvent, such as water (e.g., deionized water). For example, water(e.g., deionized water) may constitute from about 20 wt. % to about 95wt. %, in some embodiments from about 30 wt. % to about 90 wt. %, and insome embodiments, from about 40 wt. % to about 85 wt. % of theelectrolyte. A secondary solvent may also be employed to form a solventmixture. Suitable secondary solvents may include, for instance, glycols(e.g., ethylene glycol, propylene glycol, butylene glycol, triethyleneglycol, hexylene glycol, polyethylene glycols, ethoxydiglycol,dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether,ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g.,methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones(e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters(e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate,methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.);amides (e.g., dimethylformamide, dimethylacetamide,dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones);sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane);and so forth. Such solvent mixtures typically contain water in an amountfrom about 40 wt. % to about 80 wt. %, in some embodiments from about 50wt. % to about 75 wt. %, and in some embodiments, from about 55 wt. % toabout 70 wt. % and secondary solvent(s) in an amount from about 20 wt. %to about 60 wt. %, in some embodiments from about 25 wt. % to about 50wt. %, and in some embodiments, from about 30 wt. % to about 45 wt. %.The secondary solvent(s) may, for example, constitute from about 5 wt. %to about 45 wt. %, in some embodiments from about 10 wt. % to about 40wt. %, and in some embodiments, from about 15 wt. % to about 35 wt. % ofthe electrolyte.

If desired, the electrolyte may be relatively neutral and have a pH offrom about 4.5 to about 7.0, in some embodiments from about 5.0 to about6.5, and in some embodiments, from about 5.5 to about 6.0. One or morepH adjusters (e.g., acids, bases, etc.) may be employed to help achievethe desired pH. In one embodiment, an acid is employed to lower the pHto the desired range. Suitable acids include, for instance, inorganicacids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoricacid, polyphosphoric acid, boric acid, boronic acid, etc.; organicacids, including carboxylic acids, such as acrylic acid, methacrylicacid, malonic acid, succinic acid, salicylic acid, sulfosalicylic acid,adipic acid, maleic acid, malic acid, oleic acid, gallic acid, tartaricacid, citric acid, formic acid, acetic acid, glycolic acid, oxalic acid,propionic acid, phthalic acid, isophthalic acid, glutaric acid, gluconicacid, lactic acid, aspartic acid, glutaminic acid, itaconic acid,trifluoroacetic acid, barbituric acid, cinnamic acid, benzoic acid,4-hydroxybenzoic acid, aminobenzoic acid, etc.; sulfonic acids, such asmethanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid,trifluoromethanesulfonic acid, styrenesulfonic acid, naphthalenedisulfonic acid, hydroxybenzenesulfonic acid, etc.; polymeric acids,such as poly(acrylic) or poly(methacrylic) acid and copolymers thereof(e.g., maleic-acrylic, sulfonic-acrylic, and styrene-acryliccopolymers), carageenic acid, carboxymethyl cellulose, alginic acid,etc.; and so forth. Although the total concentration of pH adjusters mayvary, they are typically present in an amount of from about 0.01 wt. %to about 10 wt. %, in some embodiments from about 0.05 wt. % to about 5wt. %, and in some embodiments, from about 0.1 wt. % to about 2 wt % ofthe electrolyte.

The electrolyte may also contain other components that help improve theelectrical performance of the capacitor. For instance, a depolarizer maybe employed in the electrolyte to help inhibit the evolution of hydrogengas at the cathode of the electrolytic capacitor, which could otherwisecause the capacitor to bulge and eventually fail. When employed, thedepolarizer normally constitutes from about 1 to about 500 parts permillion (“ppm”), in some embodiments from about 10 to about 200 ppm, andin some embodiments, from about 20 to about 150 ppm of the electrolyte.Suitable depolarizers may include nitroaromatic compounds, such as2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid,3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroacetophenone,3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole,3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde,3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol,3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid,3-nitrophthalic acid, 4-nitrophthalic acid, and so forth. Particularlysuitable nitroaromatic depolarizers for use in the present invention arenitrobenzoic acids, anhydrides or salts thereof, substituted with one ormore alkyl groups (e.g., methyl, ethyl, propyl, butyl, etc). Specificexamples of such alkyl-substituted nitrobenzoic compounds include, forinstance, 2-methyl-3-nitrobenzoic acid; 2-methyl-6-nitrobenzoic acid;3-methyl-2-nitrobenzoic acid; 3-methyl-4-nitrobenzoic acid;3-methyl-6-nitrobenzoic acid; 4-methyl-3-nitrobenzoic acid; anhydridesor salts thereof; and so forth. Without intending to be limited bytheory, it is believed that alkyl-substituted nitrobenzoic compounds maybe preferentially electrochemically adsorbed on the active sites of thecathode surface when the cathode potential reaches a low region or thecell voltage is high, and may be subsequently desorbed therefrom intothe electrolyte when the cathode potential goes up or the cell voltageis low. In this manner, the compounds are “electrochemicallyreversible”, which may provide improved inhibition of hydrogen gasproduction.

The particular manner in which the components are incorporated into thecapacitor is not critical and may be accomplished using a variety oftechniques. Referring to FIG. 1, for example, one embodiment of anelectrolytic capacitor 40 is shown that includes an electrolyte 44disposed in electrical contact with an anode 20 and cathode 43. Theanode 20 contains a dielectric layer (not shown) and is in electricalcontact with a lead 42. The lead 42 may be formed from any electricallyconductive material, such as tantalum, niobium, nickel, aluminum,hafnium, titanium, etc., as well as oxides and/or nitrides of thereof.In certain embodiments, electrical contact with the anode 20 may beaccomplished by electrically coupling the lead 42 by resistance or laserwelding. The cathode 43 is formed from a metal substrate 41, such asdescribed above, and a conductive polymer coating 49. In thisembodiment, the cathode substrate 41 is in the form of acylindrically-shaped “can” with an attached lid. As shown, theconductive polymer coating 49 in this embodiment is formed on aninterior surface of the substrate 41.

A liquid seal 23 (e.g., glass-to-metal) may be employed that connectsand seals the anode 20 to the cathode 43. An electrically insulatingbushing 89 (e.g., polytetrafluoroethylene (“PTFE”)) and/or support 91may also be employed to help stabilize the anode 20 and lead 42 andmaintain the desired spacing within the capacitor. If desired, aseparator (not shown) may also be positioned between the cathode 43 andanode 20 to prevent direct contact between the anode and cathode, yetpermit ionic current flow of the electrolyte 44 to the electrodes.Examples of suitable materials for this purpose include, for instance,porous polymer materials (e.g., polypropylene, polyethylene,polycarbonate, etc.), porous inorganic materials (e.g., fiberglass mats,porous glass paper, etc.), ion exchange resin materials, etc. Particularexamples include ionic perfluoronated sulfonic acid polymer membranes(e.g., Nafion™ from the E.I. DuPont de Nemeours & Co.), sulphonatedfluorocarbon polymer membranes, polybenzimidazole (PBI) membranes, andpolyether ether ketone (PEEK) membranes. Although preventing directcontact between the anode and cathode, the separator permits ioniccurrent flow of the electrolyte to the electrodes.

Regardless of its particular configuration, the capacitor of the presentinvention may exhibit excellent electrical properties. For example, duein part to high conductivity, the capacitor of the present invention canachieve excellent electrical properties and thus be suitable for use inthe capacitor bank of the implantable medical device. For example, theequivalent series resistance (“ESR”)—the extent that the capacitor actslike a resistor when charging and discharging in an electroniccircuit—may be less than about 1500 milliohms, in some embodiments lessthan about 1000 milliohms, and in some embodiments, less than about 500milliohms, measured with a 2-volt bias and 1-volt signal at a frequencyof 120 Hz. Likewise, the capacitance may be about 1 milliFarad persquare centimeter (“mF/cm²”) or more, in some embodiments about 2 mF/cm²or more, in some embodiments from about 5 to about 50 mF/cm², and insome embodiments, from about 8 to about 20 mF/cm².

The electrolytic capacitor of the present invention may be used invarious applications, including but not limited to medical devices, suchas implantable defibrillators, pacemakers, cardioverters, neuralstimulators, drug administering devices, etc.; automotive applications;military applications, such as RADAR systems; consumer electronics, suchas radios, televisions, etc.; and so forth. In one embodiment, forexample, the capacitor may be employed in an implantable medical deviceconfigured to provide a therapeutic high voltage (e.g., betweenapproximately 500 Volts and approximately 850 Volts, or, desirably,between approximately 600 Volts and approximately 900 Volts) treatmentfor a patient. The device may contain a container or housing that ishermetically sealed and biologically inert. One or more leads areelectrically coupled between the device and the patient's heart via avein. Cardiac electrodes are provided to sense cardiac activity and/orprovide a voltage to the heart. At least a portion of the leads (e.g.,an end portion of the leads) may be provided adjacent or in contact withone or more of a ventricle and an atrium of the heart. The device alsocontains a capacitor bank that typically contains two or more capacitorsconnected in series and coupled to a battery that is internal orexternal to the device and supplies energy to the capacitor bank. Due inpart to high conductivity, the capacitor of the present invention canachieve excellent electrical properties and thus be suitable for use inthe capacitor bank of the implantable medical device.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures

All test procedures were measured in conjunction with a cylindricaltantalum anode pressed to a size of 17.3 mm (length)×7.2 mm (diameter)and weight of 4.4 g, and anodized to 10V. The anode exhibited acapacitance of 6.8 mF at a frequency of 120 Hz. The electrolyte was a5.0 M aqueous solution of sulfuric acid (specific gravity of 1.26g/cm³). The wet capacitance was determined from following formula:1/C _(wet)=1/C _(anode)+1C _(cathode)Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency was 120 Hz andthe temperature was 23° C.±2° C.

Wet Capacitance

The capacitance was measured using a Keithley 3330 Precision LCZ meterwith Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peaksinusoidal signal. The operating frequency was 120 Hz and thetemperature was 23° C.±2° C.

Storage Test

Certain electrical properties (ESR and capacitance) were determinedafter storage testing. More particularly, 10 samples were filled with5.0 M aqueous solution of sulfuric acid (specific gravity of 1.26 g/cm³)for 250 hours at ambient temperature. The samples were then tested inthe manner described above.

Temperature+Pressure Test:

Certain electrical properties (ESR and capacitance) were determinedafter temperature and pressure testing. More particularly, 10 sampleswere put into a pressure cooker filled with 5.0 M aqueous solution ofsulfuric acid (specific gravity of 1.26 g/cm³) for 100 hours at 125° C.The samples were then tested in the manner described above.

Example 1

Initially, 100 pieces of cylindrical tantalum cans with a size of 18.3mm (length)×9.1 mm (internal diameter) were sandblasted with a JetStreemBlaster II (SCM System, Inc.). The sandblasting media was black siliconecarbide grit having a size of 63 to 106 μm). The media flow rate was 0.5grams per second via a 3.2-millimeter blasting nozzle. All pieces ofcylindrical tantalum cans were sandblasted to a control level 10.7millimeters (from potential 18.3 millimeters), using appropriateferrules. The sandblasting time was 20 seconds. These samples were thendegreased in water with surfactants in an ultrasonic bath for 5 minutes,rinsed 3 times in deionized water, and then dried at a temperature of85° C. for 5 minutes. A precursor solution was thereafter applied to themicroroughened surface that contained four (4) parts by weight ofethanol (Sigma-Aldrich, Co.), 0.1 part by weight of methylpyrrolidone(Sigma-Aldrich, Co.), 1 part by weight of 3,4-ethylenedioxythiophene(H.C. Starck GmbH under the designation Clevios™ M), and 10 parts byweight of 40% butanol solution of iron(III)-p-toluene sulfonate (H.C.Starck GmbH under the designation Clevios™ C). The tantalum cans werefilled to the control level with the polymerization precursor solutionfor five (5) minutes. The cans were then drained using a vacuum for upto one (1) minute, and were then put into a drying oven for 15 minutesat 85° C. The resulting structure of poly(3,4-ethylenedioxythiophene)was washed in methanol to remove reaction by-products for five (5)minutes and the tantalum cans were put into a drying oven for five (5)minutes at 85° C. This polymerization cycle was repeated four (4) times.

Example 2

100 pieces of cylindrical tantalum cans were prepared as described inExample 1, except that the sandblasting time was 15 seconds.

Example 3

100 pieces of cylindrical tantalum cans were prepared as described inExample 1, except that the sandblasting time was 10 seconds.

Example 4

100 pieces of cylindrical tantalum cans were prepared as described inExample 1, except that the sandblasting time was 5 seconds.

Example 5

100 pieces of cylindrical tantalum cans were sandblasted as described inExample 1. Thereafter, a conductive polymer coating was formed bydipping the cans into a butanol solution of iron (III) toluenesulfonate(Clevios™ C, H.C. Starck) for five (5) minutes and subsequently into3,4-ethylenedioxythiophene (Clevios™ M, H.C. Starck) for five (5)minutes. The cans were drained using a vacuum for up to one (1) minuteand were put into a drying oven for 45 minutes at 30° C. The resultingstructure of poly(3,4-ethylenedioxythiophene) was washed in methanol toremove reaction by-products for five (5) minutes and the tantalum canswere put into a drying oven for five (5) minutes at 85° C. Thispolymerization cycle was repeated (4) times.

Once formed, 10 samples of the cathodes of Examples 1-5 were then testedfor capacitance and ESR in the manner described above. Furthermore, thecapacitance and ESR of the samples was also measured after “storagetesting” and “temperature/pressure testing” as described above. Theresults are shown below in Tables 1 and 2.

TABLE 1 Storage Testing Average (before Average Ex- testing) (aftertesting) am- CAP ESR CAP ESR ΔCAP ΔESR ΔCAP ΔESR ple (mF) (mΩ) (μF) (mΩ)(mF) (mΩ) [%] [%] 1 7.36 124 7.41 111 0.05 −13 0.7 −10.2 5 3.48 209 2.09183 −1.39 −26 −39.9 −12.4

TABLE 2 Temperature/Pressure Testing Average (before Average Ex-testing) (after testing) am- CAP ESR CAP ESR ΔCAP ΔESR ΔCAP ΔESR ple(mF) (mΩ) (μF) (mΩ) (mF) (mΩ) [%] [%] 1 7.38 123 7.42 115 0.04 −9 0.5−7.0 2 7.37 124 7.44 108 0.07 −15 1.0 −12.4 3 7.32 125 7.38 109 0.06 −160.9 −12.8 4 7.37 123 7.40 113 0.03 −11 0.4 −8.6 5 3.46 219 2.04 194−1.42 −25 −41.0 −11.4

As indicated, the capacitance values of the sequentially appliedconductive polymer coating (Example 5) decreased significantly aftertesting. In contrast, the capacitance values of the mixedmonomer/catalyst system (Examples 1-4) actually experienced a slightincrease. Without intending to be limited by theory, it is believed thatthe enhanced performance of Examples 1-4 is due in part to the excellentadhesion to the tantalum substrate exhibited by the mixedmonomer/catalyst system.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A method for forming a cathode of a wetelectrolytic capacitor, the method comprising: applying a precursorsolution to a roughened surface of a metal substrate, wherein theprecursor solution contains a substituted thiophene monomer and anoxidative catalyst and the oxidative catalyst is employed in an amountless than the stoichiometric amount; and polymerizing the substitutedthiophene monomer to form a coating that contains an intrinsicallyconductive substituted polythiophene.
 2. The method of claim 1, whereinthe substituted polythiophene has recurring units of general formula(I), formula (II), or both:

wherein, A is an optionally substituted C₁ to C₅ alkylene radical; R isa linear or branched, optionally substituted C₁ to C₁₈ alkyl radical;optionally substituted C₅ to C₁₂ cycloalkyl radical; optionallysubstituted C₆ to C₁₄ aryl radical; optionally substituted C₇ to C₁₈aralkyl radical; optionally substituted C₁ to C₄ hydroxyalkyl radical,or hydroxyl radical; and x is an integer from 0 to
 8. 3. The method ofclaim 2, wherein A is an optionally substituted C₂ to C₃ alkyleneradical and x is 0 or
 1. 4. The method of claim 1, wherein thesubstituted polythiophene is poly(3,4-ethylenedioxythiophene).
 5. Themethod of claim 1, wherein the monomer includes a3,4-alkylenedioxythiophene.
 6. The method of claim 5, wherein themonomer includes 3,4-ethylenedioxythiophene or a derivative thereof. 7.The method of claim 1, wherein the oxidative catalyst includes an ironsalt of an inorganic acid, an iron salt of an organic acid, or acombination thereof.
 8. The method of claim 7, wherein the oxidativecatalyst includes iron(III)-p-toluene sulfonate, iron(III)-o-toluenesulfonate, or a combination thereof.
 9. The method of claim 1, whereinthe precursor solution contains less than about 2 moles of the catalystper mole of the monomer.
 10. The method of claim 1, wherein theprecursor solution contains from about 0.8 to about 1.4 moles of thecatalyst per mole of the monomer.
 11. The method of claim 1, wherein theprecursor solution contains a polar protic solvent, non-polar solvent,aprotic solvent, or a combination thereof.
 12. The method of claim 1,wherein the solution is maintained at a temperature of from about −10°C. to about 15° C. prior to polymerization.
 13. The method of claim 1,wherein the metal substrate comprises titanium, tantalum, or acombination thereof.
 14. The method of claim 1, wherein the surface ofthe metal substrate is mechanically abraded.
 15. The method of claim 1,wherein the surface of the metal substrate is chemically etched,electrochemically etched, or a combination thereof.
 16. A method forforming a wet electrolytic capacitor, the method comprising: applying aprecursor solution to a roughened surface of a metal casing, wherein theprecursor solution contains a substituted thiophene monomer and anoxidative catalyst and the oxidative catalyst is employed in an amountless than the stoichiometric amount; polymerizing the substitutedthiophene monomer to form a coating that contains an intrinsicallyconductive substituted polythiophene; and placing the coated metalcasing into electrical communication with an anode and electrolyte,wherein the anode is formed from a porous anode body that contains adielectric layer.
 17. The method of claim 16, wherein the anode bodyincludes tantalum, niobium, or an electrically conductive oxide thereof.18. The method of claim 16, wherein the liquid electrolyte is aqueous.19. The method of claim 16, wherein the liquid electrolyte has a pH offrom about 4.5 to about 7.0.
 20. The method of claim 16, wherein theliquid electrolyte includes sulfuric acid.
 21. The method of claim 16,wherein the casing is generally cylindrical.